Rafael Nadal and Roger Federer's extraordinary year was unpredictable but also proof that great athletes shouldn't be questioned according to the latter's ex-coach Paul Annacone.

With the dominance of Novak Djokovic over the years and the rise of Andy Murray who had just ended 2016 as the new world number one, not many expected Nadal or Federer to win major honours again.

However, having returned in January from long lay-offs, the two long-time rivals have both upset the odds and turned back the clock with their performances this year.

The pair, as it stands, have won a combined 13 titles in 2017 and have split the four Grand Slams with each other as they currently occupy the top two places in the ATP tennis rankings.

For world number one Nadal, the Spaniard achieved his first Grand Slam since 2014 when he won a record-breaking 10th French Open title.

As for Federer, the Australian Open victory for the 36-year-old was his first Grand Slam win since 2012 as he seeks yet another title at the ATP Finals in London.

Annacone reflected on the calendar year, stating that no one could have predicted the duo's success while adding that it should serve as a warning to all the doubters.

"In 2017, we have had the privilege of watching two of the most accomplished male tennis players in history — Roger and Rafael Nadal — going toe-to-toe with Father Time and coming out ahead," Annacone wrote on the Nitto ATP Finals' official website.

"Who could have predicted this? But I circle back to my warning that questioning great athletes is a dangerous game.

"All season, I've been watching in awe as those great champions have shown us more magic, grit and passion as they rewrite the record books. I hope you've been enjoying it as much as I have."

The next event for Nadal and Federer is the season-ending ATP Finals in London, which begin on 12 November.